1. Field of the Invention
The present invention relates to a chuck and a fixing device for holding a sheet body in a state in which the sheet body is wound onto a periphery of a rotation body, and to an image recording device.
2. Description of the Related Art
In general, in printing, a photosensitive printing plate (referred to as a printing plate hereinafter) in which a photosensitive layer is formed on a support body formed of a sheet such a thin aluminum plate and the like is used. This printing plate is used as a printing plate for printing after an image is recorded due to exposure, and a development process or the like is carried out.
This type of printing plate includes one in which, in the image exposure device for carrying out image exposure, the printing plate is wound onto a rotation drum and held, then while the rotation drum is rotated at a high speed, a light beam is radiated onto the printing plate in accordance with the image data for recording on the printing plate, to thereby scan-expose the printing plate.
Image exposure devices in which a rotation drum is used to expose the printing plate include a type which uses a fixing device which, when the printing plate is wound onto the rotation drum, clamps the non-image portions of both ends of the printing plate which are along a circumferential direction of the rotation drum, between the drum and itself. In this type of a fixing device, the printing plate, generally, the clamp portion which opposes the printing plate is urged in the circumferential direction of the rotation drum by an urging force, and the printing plate is thereby nipped between the clamp portion and the rotation drum.
In this case, by the clamp portion being fit into a plurality of grooves which are provided in the circumferential direction of the rotation drum, the grooves serve as support points and the clamp portion is fixed to the rotation drum.
However, when the rotation drum is rotated at a high speed in a state in which a sheet body such as a printing plate is wound onto the rotation drum, a centrifugal force is exerted on the clamp portion causing the holding force to decrease.
It may be thought that if the number of support points is increased, the clamp portion will be firmly held onto the rotation drum, but if the number of support points is increased, the number of grooves on the rotation drum also has to be increased. If maintaining the continuity of a surface such that there are no uneven portions when winding the printing plate onto the peripheral surface of the drum is considered, the number of grooves that can be formed is limited. Thus, it becomes necessary to form a clamp that is formed of a material having high rigidity.
As a result of this, the weight is increased, and due to increase in the rotational torque, it becomes necessary to increase the driving force of the clamp. It also becomes necessary to increase the durability and the like of the support structure and this causes obstruction in other sections. It is therefore not preferable that the clamp be formed of a material having high rigidity.
In view of the above, the object of the present invention is to provide a sheet body fixing device in which when a sheet body such as a printing plate or the like is wound onto a rotation drum, the sheet body is securely fixed to the rotation drum and the coming up of the sheet off the drum surface after the fixing is kept at a minimum.
The invention according to a first aspect of the present invention is a sheet fixing device for holding a sheet in a state in which the sheet is wound around a peripheral surface of a rotation drum, the sheet fixing device comprising:
a clamp portion disposed along an axial direction at one portion of the peripheral surface of the rotation drum, at least one of a leading end portion and a trailing end portion of the sheet being nipped between the clamp portion and the peripheral surface of the rotation drum; and
a fixing element which fixes the clamp portion to the peripheral surface of the rotation drum at a plurality of support points along an axial direction of the rotation drum,
wherein when the clamp portion is fixed to the rotation drum by the fixing element, if the pitches between the support points at which the clamp portion is fixed to the rotation drum are L1 (1) . . . L1 (n) where n is an integer not less than 1, and the pitches between both end portions of the clamp portion and supports closest to the both end portions thereof are L2 (1) and L2 (2), then L1 (1) . . . L1 (n) and L2 (1)and L2 (2) are set so that the maximum deformation amounts xcex94 L1 (1) . . . xcex940 L1 (n) and xcex94 L2 (1) and xcex94 L2 (2) for the respective pitches during the rotation of the drum are substantially equal.
In the invention according to the first aspect and a second aspect of the present invention, when the rotation drum is rotated in a state in which a sheet is clamped by the clamp portion, due to a so-called centrifugal force being exerted, a force operates which causes the clamp portion to tend to come up off the surface of the rotation drum. In this case, because the supports are firmly fixed to the surface of the rotation drum, the deformation amount is insignificant, and the maximum amount of deformation is at the center of the length between the support points and is an arch-shaped deformation.
On the other hand, deformation of the length between the support points closest to the end portions and said end portions differ from the deformation of the length between the support points.
Even if deformation cannot be totally eliminated, extreme deformation of one portion in the longitudinal direction of the clamp portion (the axial direction of the rotation drum) can be avoided, and one side of the sheet (for example, the leading side) is thereby held by a substantially uniform nipping force. Thus, the coming up of the sheet from off the surface of the drum can be controlled.
In the first aspect of the present invention, as a means for avoiding extreme deformation of one portion, given that the pitches between the support points at which the clamp portion is fixed to the rotation drum are L1 (1) . . . L1 (n) where n is an integer not less than 1, and the pitches between both end portions of the clamp portion and support points closest to the both end portions is L2 (1) . . . L2 (2), then L1 (1) . . . L1 (n) and L2 (1) and L2 (2) are set so that the maximum deformation amounts xcex94L1 (1) . . . xcex94L1 (n) and xcex94L2 (1) and xcex94L2 (2) for the respective pitches during the rotation of the drum are substantially equal.
Further, the second aspect of the present invention is the invention according to the first aspect of the present invention, wherein L(1) . . . L1(n) are all set to be substantially equal to L1, and L2(1) and L2(2) for both end portions are set to be substantially equal to L2, and one of L1 and L2 is set, with the other of L1 and L2 as a reference, such that a deformation amount xcex94L1 of L1 and deformation amount xcex94L2 of L2 are substantially equal.
As a result, the deformation amounts xcex94L1 between the support points are made substantially equal, and in order for xcex94L1, as well as the deformation amount xcex94L2 which occurs between a support point and one end portion (L2) to be made substantially the same, one of L1 and L2 is set with the other of the two as a reference. That is, even for clamps having the same amount of support points, by causing the relationship between L1 and L2 to be fixed, the maximum deformation amount is reduced.
The invention according to a third aspect of the present invention is the invention according to the second aspect of the present invention, wherein a dimension L2 between a support point and an end portion, is set by using a dimension L1 between support points, as a reference.
According to the third aspect of the present invention, by first setting L1 which is the dimension between the support points, adjusting L2 which is the dimension between a support and an end portion becomes comparatively easy, and thus the desired conditions can be easily obtained.
The invention according to a fourth aspect of the present invention is the invention according to the second aspect of the present invention, wherein the ratio (L2/L1) of the dimensions L2 between a support point and an end portion L1 to the dimensions between the support points, is plotted on the horizontal axis and the respective deformation amounts are plotted on the vertical axis, and the L2/L1 value at the point where the characteristic curves for the deformation amounts of L1 and L2 respectively intersect is made the set value.
According to the fourth aspect of the present invention, L2/L1 which is a relational expression of L1 and L2 is plotted on the horizontal axis and the deformation amounts between the support points as well as the deformation amounts between a support point and an end portion are plotted on the vertical axis. Curves are obtained in which as the dimension for one (of L1 and L2) increases, the deformation amount of said one increases. The point at which the two curves intersect is selected as the optimum point (the point of compromise between the two dimensions). The dimensions L1 between the support points and the dimensions L2 between a support point and an end portion are thus easily set.
The invention according to a fifth aspect of the present invention is the invention according to the second aspect of the present invention, wherein in a case in which there are three or more support points, at least one length between support points is selected.
According to the fifth aspect of the present invention, in the case where there are three or more support points, the dimension of at least two support points are used as the dimension between the support points. In such cases, since the dimensions between the support points are substantially equal, basically, one support point may be selected.
By the way, even if the dimension between the support points are substantially equal, depending on whether the number of support points is an odd number or an even number, the deformation amounts may differ. It may be thought that this difference is caused by whether or not a support point is located at the center of the clamp portion in the longitudinal direction thereof. That is to say, when there is a plurality of support points, for example if there are three, because the other two support points are separated from the support point at the center of the clamp portion by the same length L1, the deformation amounts for both lengths are the same. On the other hand, when there is a plurality of support points, for example four support points, the center of the length between the support points is at the position of the center of the clamp portion in the longitudinal direction and thus the conditions at that length between the support points, which length includes the center of the clamp portion, differ from the conditions at the lengths between the other support points. As a result the deformation amounts may also differ. In such cases, the lengths between the support points which include the center of the clamp portion, as well as the lengths between one of those support points and the other two support points are selected as L1 and these, along with the lengths L2 determine the conditions for the least amount of deformation. For example, using a relational expression such as that of the third aspect of the present invention, the point of compromise of the three curves is determined.
The invention according to a sixth aspect of the present invention is the invention according to the fourth aspect of the present invention, wherein 0.37 less than L2/L1 less than 0.43.
According to the sixth aspect of the present invention, the L2/L1 value is based on the results of experiments (See the Detailed Description of the Invention.) and by L2/L1 being set in this range, high speed rotation of the rotation drum becomes possible.